The invention is directed to a method for instantaneously monitoring two or more wavelengths of light and an apparatus for effecting the same. More particularly, the invention is an acousto-optic dispersive light filter (AODLF), which is an electronically adjustable spectroscopic device capable of instantaneously monitoring many wavelengths with a fixed RF drive frequency. The present invention has approximately a one octave optical range, the center of which is selected by changing the RF drive frequency. The resolution of the AODLF in the infrared is several thousand, and is also electronically adjustable. The process and apparatus of this invention is particularly useful for the detection and analysis of, for example, short light pulses.
The acousto-optic tunable filter (AOTF) has long been recognized as an effective way to rapidly analyze light with an all solid-state device, over a large spectral bandwidth. Among the advantages of the AOTF are its large angular aperture with good spectral resolution, making it well suited for applications both in the visible and infrared ranges. However, the AOTF is inherently a single-channel device; i.e., in its natural mode of operation, only one wavelength resolution element at a time may be passed, although it is possible to randomly access such elements in times typically on the order of tens of microseconds. This random access time is simply limited by the travel time of the acoustic wave across the optical aperture. Thus, the filtered light falls upon a single detector element, and the spectrum scanning is performed by a linear time sweep of the applied RF power. A typical application of an AOTF as described above is set forth in detail in allowed U.S. patent application Ser. No. 345,123 filed Feb. 2, 1982 and entitled "An Automated Acousto-Optic Infrared Analyzer". This allowed patent application is assigned to the assignee of the subject patent and is incorporated by reference as if set forth in full herein. Such a spectrum scanning operation is satisfactory in the presence of cw or very slowly varying light signals, in which the composition of the light remains essentially constant over a complete sweep duration of the RF range. Complications, however, may arise in the presence of pulsed-light signals, which will go undetected unless the proper acoustic tuning frequency is present in the optical aperture at the instant of the light pulse. Thus, if it is desired to detect the presence of a known optical wavelength, but at an unknown time of arrival, the AOTF must be continuously excited with RF of exactly that frequency. This may be extended to a few wavelengths simultaneously, for which frequencies corresponding to each of the wavelengths must be simultaneously applied to the AOTF. In this case, the determination of which of the several wavelengths may have been detected will require a strategy of frequency dropping over a few pulses. Operation at reduced duty cycle will result in lower detection probability. These difficulties make it desirable to consider possible acousto-optic techniques which are many channel in nature, so that the device will be open to receive all of the wavelength resolution elements simultaneously.
It is, therefore, an object of this invention to provide an acousto-optic dispersive light analyzer which is an electronically adjustable spectroscopic device capable of instantaneously monitoring many wavelengths with a fixed drive frequency and a technique for operating such an acousto-optic device to obtain the aforedescribed results.
The AODLF is functionally very similar to a fixed grating, but there are several important differences which are advantageous in certain applications. The two principal differences are the tunability of the AODLF and its birefringent operation. A conventional grating, being optically isotropic (i.e., no change of polarization of the diffracted light), must be blazed in order to concentrate the diffracted light into a single order; however, the birefringence of the AODLF produces only a single order. Because the fixed grating is isotropic, the angular aperture with a blaze will be more limited than the angular aperture of the AODLF and correspondingly the optical range will also be more limited. A large change in the optical bandcenter of the fixed grating requires a mechanical change in the angle of incidence, while a bandcenter change of the AODLF is accomplished simply by a change in the RF only, with no change in the angle of incidence. Thus, the unique feature which is provided by the AODLF of this invention is the electronic tunability of the grating constant, which allows enhanced flexibility of operation, such as large changes of spectral range. The electronic tunability also easily permits the frequency modulation of the optical signal in order to perform derivative spectroscopy, which may improve the signal-to-noise ratio over that of a constant signal.
It is known that the simplest AO Bragg diffraction of the type used in scanning and deflection can be employed, in principle, to effect a light spectrum analyzer. However, there are serious limitations to this simplistic approach. Ideally, such a device would work in the following fashion. The light to be analyzed is incident on the cell at some fixed angle which will not be varied over the entire spectral range, and with some usably large angular aperture; the acousto-optic cell will be excited with a fixed cw RF so that there will be unity probability of intercept, and at as near 100% efficiency as possible for maximum sensitivity; each wavelength resolution element should emerge from the cell at a different diffracted angle, so that a detector array in the focal plane of the system may be used to analyze the light spatially; the spectral range and the resolution should be electronically controllable so that no mechanical motion is needed to make adjustments for its operation.
In the simplest configuration, the low frequency or Raman-Nath mode of diffraction, the relationship between the optical wavelength, .lambda., the acoustic wavelength, .LAMBDA.=v/f, and the interaction length, l, must satisfy EQU 4l.lambda./.LAMBDA..sup.2 .ident.Q&lt;&lt;1, (1)
where v is the acoustic wave velocity and f its frequency. When this condition is satisfied, for light incident normal to the acoustic wave propagation direction, light is diffracted at the Bragg angle .theta..sub.B into multiple positive and negative orders, n, according to EQU sin n.theta..sub.B =n.lambda./.LAMBDA.=n.lambda.f/v (2)
The resolution of the cell is simply given by N, the number of acoustic wavelengths within the optical aperture, L, so that EQU N=L/.LAMBDA.=Lf/v (3)
and the angular aperture, A, is given by the acoustic wave diffraction spread EQU A=.LAMBDA./l=.lambda./l sin .theta..sub.B ( 4)
A few characteristics can be easily evaluated for the two favorable infrared materials, Tl.sub.3 AsS.sub.3 (TAS) and Hg.sub.2 Cl.sub.2, at an optical wavelength of, say, 5 .mu.m. To satisfy the condition of Q&lt;1 we take l=1 cm for TAS, and 0.5 cm for Hg.sub.2 Cl.sub.2, and an RF of 2.3 MHz for the former and 1.1 MHz for the latter. At such low frequencies, acoustic attenuation is not a limiting factor, and we may make the optical aperture crystal size limited, said 5 cm. The number of resolution elements will then be 110 for TAS and 158 for Hg.sub.2 Cl.sub.2. These characteristics are summarized in Table I.
TABLE I ______________________________________ CHARACTERISTICS OF RAMAN-NATH AO DISPERSION CELL .lambda. = 5 .mu.m, L = 5 cm Material 1(Cm) f(MHz) .theta..sub.B (deg) A(deg) N ______________________________________ TAS 1 2.3 0.63 2.6 110 Hg.sub.2 Cl.sub.2 0.5 1.1 0.91 3.6 158 ______________________________________
There are some serious limitations on utilizing Raman-Nath diffraction in this fashion for optical spectrum analysis. First, since the interaction length l, must be small, it will not be possible for the efficiency to be high at long infrared wavelengths. This is because the RF power requirements increase with the square of the wavelength, and the above values of l will not be large enough even with these efficient materials; compounding this difficulty is the wasting of diffracted light to orders other than the +1, so that the detected light efficiency at a single order can never be high. Second, the presence of multiple diffraction orders restricts the optical bandwidth to one octave, in order that there be no overlap between first and second orders. Third, the Bragg angles are small in comparison with the angular aperture, so that it will be necessary that the input light be highly collimated.
One might hope to avoid these difficulties by operating in the Bragg diffraction mode, for which it is required that the parameter Q&gt;&gt;1. To illustrate the problems that approach leads to, let us assume a factor of 10 increase in RF, for the same values of l. Operation in the Bragg regime requires that the light to be analyzed be incident to the acoustic wave at the Bragg angle, so that the incident light angle must therefore vary with the optical wavelength. If we require the incident light angle to be fixed, the spectral range will be limited by the angular aperture. Some of these considerations are summarized in Table II.
TABLE II ______________________________________ CHARACTERISTICS OF BRAGG AO DISPERSION CELL .lambda. = 5 .mu.m, L = 1 cm Spectral Material 1(Cm) f(MHz) A(deg) Range(.mu.m) N ______________________________________ TAS 1 23 0.26 0.21 220 Hg.sub.2 Cl.sub.2 0.5 11 0.36 0.20 316 ______________________________________
It is apparent that this approach is unsatisfactory due to the very small value of spectral range that results from the small angular aperture. The range can be enlarged by mechanical rotation of the cell to match the Bragg angle as the optical wavelength is changed, but then the device is no longer purely electronic.
The acousto-optic tunable filter (AOTF) which is known generally consists of a transducer plate and a transparent optical medium through which acoustic waves generated by the transducer propagate. The transducer is typically a thin plate of a piezoelectric crystal such as lithium niobate (LiNbO.sub.3) or quartz (SiO.sub.2). The optical medium must be crystalline and possess the appropriate symmetry properties, such as thallium arsenic selenide (Tl.sub.3 AsSe.sub.3). The transducer is operably associated with the optical medium by a bond of high acoustic quality. The operational concept of the AOTF is explained in detail in the article "Tunable Acousto-Optic Filters and Their Application to Spectroscopy", Feichtner, J. D., et al., SPIE Vol. 82, page 106 (1976), the contents of which are incorporated herein by reference.